The relationship between reinforcer amount and daytime smoking reduction in smokers offered money for reduced afternoon breath carbon monoxide (CO) levels was examined. Twenty-three hired regular smokers with average baseline CO levels of about 30 ppm were exposed in random order to five sliding scale payment schedules that changed daily or weekly. Money was available for afternoon CO readings between 0 and 21 ppm with pay amount inversely related to the absolute CO reading obtained. Maximum pay amount for readings below 7 ppm varied among $0, $1.50, $3, $6, and $12 per day. Contingent reinforcement promoted CO and daytime cigarette reduction within individuals with the amount of behavior change related to the amount of payment available. Average CO levels decreased from 30 to 15 ppm as a function of pay amount whereas self-reported daytime cigarettes decreased from 12 to 5 per day. Average minutes of cigarette abstinence prior to the afternoon study contact increased from 62 to 319 minutes as a function of pay amount, whereas the percentage of available money earned increased from 22% to 48%. Nontargeted evening cigarette use also decreased during periods of daytime smoking reduction. The orderly effects of this contingent reinforcement intervention on daytime smoking of regular smoker volunteers suggest that this is a sensitive model for continued evaluation of factors that influence smoking reduction and cessation.DESCRIPTORS: smoking, reinforcement, contingencies, carbon monoxide reduction Two recent studies by Bigelow (1982, 1983) Thompson, 1981;Russell, Wilson, Patel, Feyerabend, & Cole, 1975 Widdowson, & Hulley, 1977) and reflect within-subject changes in smoke exposure (Henningfield, Stitzer, & Griffiths, 1980;. Nonsmokers generally have breath CO levels below 8 ppm, whereas smokers typically have levels between 15 and 40 ppm, though these may range higher depending on the amount of recent smoke exposure.In the first study that used breath CO as a measure of recent smoking and a target for contingent reinforcement intervention , hired regular smokers were offered $5 per day to reduce their afternoon CO levels by 50% or more from their own baselines. The study showed that smokers could successfully reduce afternoon CO levels to earn payments and that they did so by reducing the number of daytime cigarettes smoked and abstaining for several hours prior to the afternoon CO measurement. A second 477 1984517. 477-483 NUMBER4 (wiNm 1984)